1. Field of the Invention
The present invention relates to a signal generating circuit that receives a current signal to generate a signal based on the current signal. The present invention also relates to an optical pickup device provided with such a signal generating circuit.
2. Description of Related Art
An optical pickup device includes a signal generating circuit. The signal generating circuit receives the output current of a light-sensitive element that receives light reflected from an optical disk, and then generates a servo error signal based on the current signal.
In an optical pickup device incorporated in a CD playback apparatus or the like, the anode of a photodiode used as a light-sensitive element is grounded so as to increase the sensitivity of the light-sensitive element to light of long wavelengths (e.g. red light). FIG. 6 shows an example of the configuration of a signal generating circuit that receives an output current of a photodiode having the anode thereof grounded and generates a servo error signal.
The signal generating circuit 11 shown in FIG. 6 includes an operational amplifier 12, a resistor 13, and a signal processing circuit 14. The signal generating circuit 11 shown in FIG. 6 receives a current outputted from a photodiode 15 whose anode is grounded. The output current of the photodiode 15 is converted into a voltage by a current-to-voltage converter formed by the operational amplifier 12 and the resistor 13. The signal processing circuit 14 receives the voltage outputted from the current-to-voltage converter, and then generates a servo error signal based on the received voltage.
On the other hand, in recent years, much effort has been spent in the development of DVD playback apparatuses and the like that use blue laser light. In an optical device incorporated in such apparatuses, a constant voltage Vcc is fed to the cathode of a photodiode used as a light-sensitive element so as to increase the sensitivity of the light-sensitive element to light of short wavelengths. FIG. 7 shows an example of the configuration of a signal generating circuit that receives an output current of a photodiode receiving a constant Vcc at the cathode thereof and generates a servo error signal. In FIG. 7, such components as find their counterparts in FIG. 6 are identified with common reference numerals.
The signal generating circuit 16 shown in FIG. 7 includes an operational amplifier 12, a resistor 13, and a signal processing circuit. 17. The signal generating circuit 16 shown in FIG. 7 receives a current outputted from a photodiode 18 that receives a constant Vcc at the cathode thereof. The output current of the photodiode 18 is converted into a voltage by a current-to-voltage converter formed by the operational amplifier 12 and the resistor 13. The signal processing circuit 17 receives the voltage outputted from the current-to-voltage converter, and then generates a servo error signal based on the received voltage.
The signal generating circuit 11 shown in FIG. 6 and the signal generating circuit 16 shown in FIG. 7 differ from each other in the polarity of the input current, and accordingly in the polarity of the voltage outputted from the current-to-voltage converter formed by the operational amplifier 12 and the resistor 13. This gives the signal processing circuit 14 and the signal processing circuit 17 different circuit configurations.
As a result, whereas the signal generating circuit 11 shown in FIG. 6 cannot process the output current of a photodiode that receives a constant voltage at the cathode thereof, the signal generating circuit 16 shown in FIG. 7 cannot process the output current of a photodiode whose anode is grounded. That is, quite inconveniently, which type of signal generating circuit to use needs to be decided according to how a photodiode is connected.